1. Technical Field
The present invention relates to a member used for a liquid crystal display, such as a backlight device (a backlight), and to a liquid crystal display using the member.
2. Related Art
In recent years, low-profiling, cost reduction, and power reduction of liquid crystal television sets (hereinafter referred to as liquid crystal TVs) with a large screen have been promoted, and widespread thereof is now in progress. In order for further low-profiling such large-screen liquid crystal TVs, super low-profiling of the backlight devices needs to be achieved. As a conventional backlight device for such large-screen liquid crystal TVs, a direct backlight shown in FIG. 2 is used.
The direct backlight shown in FIG. 2 is provided with a plurality of linear light sources 1 (fluorescent bulbs such as cold-cathode tubes) as a light source, a reflecting plate 2 disposed immediately beneath the plurality of linear light sources, a diffusing plate 3 for reducing luminance variation disposed immediately above the plurality of linear light sources, and, according to needs, a plurality of optical sheets 4 for achieving improvement of the front luminance or reduction of luminance variation of a liquid crystal display panel disposed also immediately above the plurality of linear light sources.
However, the cold-cathode tubes as the light sources 1 have tube surfaces with high luminance, and therefore, have a problem that in the case in which the distance between the light sources 1 and the diffusing plate 3 shown in FIG. 3 is short, there is caused the luminance variation on a screen of the liquid crystal display panel (not shown) mounted on the backlight to degrade the display quality. Therefore, it is necessary to assure the distance between the light sources and the diffusing plate as large as at least about the distance between the light sources, which makes it difficult to achieve the super low-profiling.
Incidentally, as a medium or small size liquid crystal display such as a laptop personal computer (hereinafter referred to as a notebook PC) or a mobile information terminal, there is used a side edge backlight shown in FIG. 4 as described in JP-A-4-162002. In the side edge backlight shown in FIG. 4, a linear light source (a fluorescent bulb such as a cold-cathode tube) or a plurality of point light sources (e.g., light emitting diodes) is used as the light source 1, and is disposed so as to be opposed to an end face (a side face) of a light guide plate 5 made of a translucent material. There are provided light diffusing dots 6 for reflecting, transmitting, and diffusing the light propagated inside thereof formed on the lower surface of the light guide plate, a reflecting plate 2 for reflecting the light disposed below the lower surface of the light guide plate, a diffusing sheet 7 having a sheet like shape for reducing the luminance variation disposed above the upper surface (a surface opposed to the liquid crystal display panel not shown) of the light guide plate, and, according to needs, a plurality of optical sheets 4 for improving the front luminance of the liquid crystal display panel disposed also above the upper surface of the light guide plate.
In the side edge backlight, since equalization of the luminance of the light emitted to the principal surface of the liquid crystal display panel can be achieved using the light guide plate, there is an advantage that the low-profiling is easier compared to the direct backlight.
Conventionally, as the light diffusing dots (hereinafter described as dots) for reflecting, transmitting, and diffusing the light in the light guide plate, there are formed white printed dots with a diameter of about 0.4 through 1.5 mm, machine-worked dots with a diameter of about 0.02 through 0.1 mm, or the like on the principal surface of the light guide plate. However, it has proved that it is very difficult to increase the light guide plate in size while keeping the small thickness by redesigning the light guide to have a diagonal size of 32-inch wide screen (the length of the light guide plate of about 720 mm) and a thickness of no greater than 3 mm in order for using the side edge backlight for a large-sized liquid crystal TV for the following reasons.
In the case with the normal white printed dots, since the ratio of “[light guide plate length]/[light guide plate thickness]” increases, it is necessary to significantly reduce the amount of light taken out by the dots in the light guide plate, in particular in the vicinity of the light source. Therefore, it is required to dramatically down-sizing the printed dots of the light guide plate in the vicinity of the light source. However, the printed dots are difficult to down-size, and cause a phenomenon that the dots become visualized, namely dots-visualization on the screen of the liquid crystal display panel in the vicinity of the light source in accordance with the down-sizing of the dots and significant drop in the coverage of the principal surface of the light guide plate caused by the down-sizing of the dots.
The machine-worked dots or the like are small in size, and therefore, the problem of dots-visualization is hard to occur. However, it is required to provide fine dots to a metal mold for forming the light guide plate corresponding the screen size of the large screen liquid crystal TV, and therefore, it is extremely difficult to manufacture the metal mold.